This invention relates to an antiskid brake control method for controlling the skidding of wheels of an automotive vehicle by repeatedly decreasing and increasing brake hydraulic pressure at the time of braking. More particularly, the invention relates to an antiskid brake control method for reducing movement of a brake pedal during antiskid brake control.
In general, antiskid brake control involves detecting skidding of a wheel at braking, eliminating skidding when this is detected by reducing the braking force acting upon the wheel, and then subsequently increasing the braking force, thereby stabilizing steering of the vehicle and making the braking distance as short as possible.
One example of an antiskid brake control apparatus for performing such brake control is of the kind shown in FIG. 4, which comprises an add-on four-channel, four-solenoid system.
As illustrated in FIG. 4, the brake circuit to which this antiskid brake control apparatus is applied is formed as a dual-system arrangement comprising a first brake system A and a second brake system B. Since the first and second brake systems A, B have the same construction and operate in the same manner, the first brake system A will be described below and a description of the second brake system B is deleted.
In the antiskid brake control circuit shown in FIG. 4, a case will be considered in which antiskid brake control as performed conventionally is applied. Under ordinary conditions, flow-control valves 6, 7 each comprising a mechanical valve and passage changeover valves 8, 9 each comprising a solenoid valve are in the states illustrated in FIG. 4. Accordingly, brake hydraulic pressure produced in an oil chamber 1a of a master cylinder 1 at the time of braking is introduced to front-and rear-wheel cylinders 2, 4 through supply passages 3, 5 and the valves 6, 7, 8, 9. As a result, braking is applied to a right front wheel FR and a left rear wheel RL.
If, by way of example, wheel velocity V.sub.wf of the right front wheel FR attains a first threshold value .alpha., which is decided based upon quantities related to wheel skidding, such as slip factor and wheel deceleration, and it is detected that the right front wheel FR is in a skidding state, the solenoid of the first passage changeover valve 8 is energized by the detection signal. As a result, the position of the first passage changeover valve 8 is changed over to shut the supply passage 3 communicating the oil chamber 1a and the front-wheel cylinder 2 and communicate the front-wheel cylinder 2 with a low-pressure accumulator (sump) 10. Consequently, the brake fluid inside the front-wheel cylinder 2 flows out to the low-pressure accumulator 10 so that there is a decline in the brake hydraulic pressure of the front-wheel cylinder 2, as illustrated in FIG. 1(b).
The brake fluid which has flowed out to the low-pressure accumulator 10 is fed under pressure by a piston pump 11 and caused to circulate to the oil chamber 1a of the master cylinder 1. By virtue of this operation, the pressure in oil chamber 1a rises and the brake pedal 1b is forced back a slight amount. The driver thus physically senses the fact that antiskid brake control is being carried out. In this case, the pulsation of pump 11 is absorbed by a volume chamber 12 and suppressed by a brake-fluid circulation suppressing orifice 13 so that almost no pulsation is transmitted to the oil chamber 1a.
The brake fluid circulated by the pump 11 acts as pilot pressure upon ports 6a, 7a of the respective first and second flow control valves 6, 7. Since the brake hydraulic pressure of front-wheel cylinder 2 has diminished, the first flow control valve 6 shuts the supply passage 3 owing to the pilot pressure. However, since the brake hydraulic pressure of the rear-wheel cylinder 4 has not declined, the second flow control valve 7 does not shut the supply passage 5 owing to the pilot pressure.
The wheel velocity V.sub.wf of the right front wheel FR continues to fall since the braking torque exceeds the rotational torque of the wheel at the start of decay. However, when the hydraulic pressure falls below a predetermined value, the rotational torque of the wheel surpasses the braking torque, whereby the wheel velocity V.sub.wf begins to recover. At this time, the brake hydraulic pressure continues to decline. In particular, as indicated by the solid line in FIG. 1(b), brake hydraulic pressure often becomes substantially zero in case of road having a low .mu..
When the wheel velocity V.sub.wf has recovered to a considerable degree and attains a second threshold value .beta. decided based upon a quantity related to vehicle skidding, the first passage changeover valve 8 is changed over to a position at which it communicates the front-wheel cylinder 2 and the oil chamber 1a. Consequently, the brake fluid circulated by the pump 11 is supplied to the front-wheel cylinder 2 while its flow rate is controlled by an orifice 18. The brake hydraulic pressure of the front-wheel cylinder 2 rises again, as indicated by the solid line in FIG. 1(b), and braking force gradually increases.
Thus, antiskid brake control control is carried out owing to a repetitive decrease and increase in the brake hydraulic pressure.
An antiskid brake control operation similar to that described above is performed when the left rear wheel RL skids and also when the left front wheel FL and right rear wheel RR of the second system B skid.
The relationship between brake hydraulic pressure and amount of consumed fluid at vehicle braking generally is as shown in FIG. 5. Specifically, whereas the amount of fluctuation in fluid consumption is small and the amount of fluctuation in brake hydraulic pressure is large for a road surface having a high coefficient of friction (a high .mu.), the amount of fluctuation in brake hydraulic pressure is small and the amount of fluctuation in fluid consumption is large for a road surface having a low coefficient of friction (a low .mu.). As a consequence, for a road having a low .mu., the amount of fluid consumption is large even if the fluctuation in hydraulic pressure is small. A problem which results is a large amount of movement of the brake pedal 1b, as indicated by the solid line in FIG. 1(c).